This invention relates to sonar transducers and, more particularly, to a transducer probe assembly adapted for detection of short pulses of high frequency sonic energy as is utilized in medical ultrasonics.
Transducers for the conversion of sonic energy into electrical signals are employed aboard ships for the detection of underwater sounds, and also in probes for the detection of sounds propagating within the tissues of living beings. The transducers are generally reciprocal devices in that electrical signals applied to the transducers are converted into sound waves which can propagate within the ocean and within living tissues.
In the past, transducers have generally been constructed of a piezoelectric ceramic material such as lead-zirconatetitanate (PZT). PZT is relatively dense as compared to water and has a much higher acoustic impedance than does water. In sonar applications, the transducer has generally been mounted in strong metallic casings with relatively heavy weights mechanically coupled to the transducer in order to provide a measure of impedance matching between the impedances of the transducer and of the water. Probes employing smaller quantities of PZT and utilizing reduced weight of the impedance matching structures have been employed in medical ultrasonic research for the observation of sound waves within living tissues. However, such probes have not been completely satisfactory due to the large difference of impedance between the transducer and the living tissue, the living tissue having an acoustic impedance substantially equal to that of water. The large difference of impedance has reduced the efficiency of conversion of sonic energy to electric energy thereby reducing the sensitivity of the probe. Furthermore, the ceramic materials utilized in transducers, in combination with their mechanical acoustic matching structures, provide a structure which is sufficiently resonant acoustically to inhibit the measurement of sonic pulses of relatively short durations, less than a microsecond duration, as are advantageously utilized in medical ultrasonics.
With a view towards providing transducers which are capable of receiving the foregoing submicrosecond sonic signals with minimal distortion, consideration has been given to a material, other than the ceramics, which has piezoelectric properties and an acoustic impedance more nearly equal to that of water than is the impedance of the ceramics. One such material, polyvinylidene fluoride is commercially available from the Penwalt Corporation of King of Prussia, Pa. and EMI Limited, Middlesex, England. However, this polymeric material is presently obtainable only in thin films, typically 30 microns thick. The films are produced with metallized layers on the top and the bottom surfaces by a deposition of, typically, aluminum on the surfaces. A problem arises in that the physical structures which have been utilized in the fabrication of sonar transducers employing the relatively massive, rugged ceramic materials do not admit the use of the relatively light, fragile polymeric film.